Recursive estimation in digital distance relaying system

A method of digital distance relaying on a three-phase electric power transmission line comprising the steps of

• successively simultaneously sampling three voltage waveforms and at least three current waveforms from said line;
• electronically detecting the existence of a fault on said line from data from said sampling;
• providing first, second, and third calculating means;
• recursively automatically computing by said first calculating means in response to each postfault sampling of each of said three voltage waveforms, three corresponding voltage waveform state estimates by means of an estimation procedure having at least two state variables for each said voltage waveform state estimate;
• recursively automatically computing by said second calculating means in response to each postfault sampling of each of two of said current waveforms, two corresponding current waveform state estimates by means of an estimation procedure having at least three state variables for each said current waveform state estimate;
• recursively automatically computing by said third calculating means in response to each postfault sampling of at least the third of said at least three current waveforms, a corresponding current waveform state estimate by means of an estimation procedure having at least three state variables for said third current waveform state estimate;
• and
• electronically determining whether said voltage and current waveform state variable estimates indicate that said fault lies within a predetermined zone and generating a circuit breaker tripping signal when said estimates so indicate.

In a distance relay having circuit breaker means for isolating a fault in one section of an alternating voltage transmission system,

• means having a first pair of input terminals energized by the voltage of said line and a second pair of input terminals energized by the current of said line, for providing periodic samples of said voltage and said current to calculating means;
• said calculating means being effective to determine the existence of the fault in response to an onset of the fault somewhere in said section and to provide an initial set of estimates including (A) cosine waveform state variable estimates respectively for said voltage and said current occurring subsequent to the onset of said fault, (B) negative sine waveform state variable estimates respectively for said voltage and said current occurring subsequent to the onset of said fault, and (C) at least one transient waveform state variable estimate for said current occurring subsequent to the onset of said fault,
• said calculating means further being effective to provide an updated set of said estimates all of said state variables during the time period between each pair of consecutive ones of said periodic samples subsequent to the onset of said fault on the basis of the most nearly previous sample of said voltage and the most nearly previous sample of said current and the most nearly previously provided set of said estimates, said calculating means being able to so provide an updated set of said estimates at least twice during the first half cycle after the onset of said fault, utilizing as to sampling data for computing said estimates only samples of said voltage and said current subsequent to the onset of said fault,
• said calculating means being effective to utilize the cosine waveform state variable estimates and the negative sine state variable estimates so updated in determining whether said fault is located in said one section of said transmission system and to trip said circuit breaker means substantially only when said fault is determined to be located in said one section.